The present invention relates to the construction of aerospace vehicle fuselages, and more particularly, to fuselage assemblies that include laminate skins having alternating metal and non-metal panels.
The use of laminate panels in aerospace construction is advantageous as the laminate panels typically have a high strength and a relatively low weight. One problem encountered with laminate panels, however, is the limited commercial availability of large laminate panels. U.S. Pat. No. 5,429,326 by Garesche et al. discloses a system for splicing laminate subpanels to make larger laminate panels for use in an aircraft fuselage. As shown in FIGS. 3A and 3B of Garesche et al., a spliced laminate panel 20 includes alternating metal layers 50 and adhesive layers 51. The metal layers are made of sections separated by spaced splice lines 55, 56, 57 and 58. Ostensibly, the spacing between the splice lines improves the strength of the final assembled panel. The aircraft fuselage includes a support structure comprised of longitudinally extending stringers 24 supported by circumferentially extending frame members 22. The laminate panels are attached to the stringers and frame members so as to form a skin, as shown in FIG. 1 of Garesche et al. The laminate panels are attached to the stringers using rivets 71, 72 that transfixes both the stringer and the panel, as shown in FIG. 7 of Garesche et al.
Although the splicing system disclosed by Garesche et al. has excellent strength characteristics, improvements in the structural strength for aircraft fuselages are always highly desired. U.S. Pat. No. 5,951,800 to Pettit discloses a splice that includes a plurality of splice straps 20 layered over the staggered splice lines so as to provide local reinforcement for the splice joint. As shown in FIGS. 1-3 of Garesche et al., the splice straps are solid sheets of metal that overlie the outermost abutting metal sheets of the laminate structure. The splice straps have sufficient width to exceed the staggered offset between all of the breaks within the splice structure. Thus, the splice straps provide further improvement in the structural strength of the splices used to construct the large laminate sheet for an aircraft fuselage.
Although some types of fasteners can be used with the large laminate panels, as described by Garesch et al., aircraft manufacturers have relied mostly on bonding for attachment of the laminate skin to the underlying frame and stringer assembly of the fuselage. Reliance on bonding over the use of fasteners is most likely due to concerns about compromising the structural strength of the spliced laminate with the insertion of fasteners. Bonding processes are generally problematic due to the need to anodize the metal being bonded and due to uneven process control during application of the adhesive. In addition, there has been a tendency to avoid placing cutouts through the laminate skins, such as for the insertion of windows, that has led to a preference for limited use of the laminate skins on the fuselage. However, limited use of laminate skins results in xe2x80x9cmixed joints, which are joints between the laminate skin and the solid metal skin. It is typically difficult to construct such mixed joints due to the different materials of the laminate and solid metal skins.
Therefore, it would be advantageous to have a system and method that allows greater employment of laminate materials in an aircraft fuselage so as to improve the strength and reduce the weight of the fuselage. In particular, it would be advantageous to have an aircraft fuselage that includes laminate panels used in areas with a large number of cutouts. Further, it would be advantageous if the laminate panels could be connected to the underlying stringers and frame members in such a way as to improve the structural integrity of the finished fuselage.
The present invention addresses the above needs and achieves other advantages by providing a fuselage comprising a skin assembly including an outer, laminate skin bonded to an inner, aluminum doubler. The fuselage also includes a support structure comprising a plurality of longitudinal stringer members and a plurality of annular frame members that are attached to, and cooperate to support, the skin assembly. Advantageously, the aluminum doubler provides additional structural support for the fuselage, and in particular, for the outer laminate skin of the skin assembly. The additional structural strength added by the aluminum doubler allows the use of an improved range of fasteners, such as knife-edge, countersink rivets and further allows the use of the laminate layer even in areas with a large number of cutouts, such as the window track of the fuselage. In addition, the members of the support structure may be interconnected via a plurality of integral flanges, which, when combined with the skin, provide improved structural strength for the entire fuselage.
In one embodiment, the present invention includes an assembly combining a collection of individual parts into a low weight but high strength fuselage for an aircraft. The fuselage assembly includes a plurality of longitudinal stringer members, a plurality of annular frame members, a lightweight doubler and a laminate sheet. The longitudinal stringer members are radially spaced from, and extend generally parallel to, the major longitudinal axis of the fuselage. Further, the longitudinal stringer members are spaced circumferentially from each other. Each of the longitudinal stringer members has a stringer wall structure that includes an outer longitudinal surface. The annular frame members are spaced along the longitudinal axis. Each of the frame members includes a frame wall structure having a plurality of outer circumferential surfaces. Each of the outer circumferential surfaces is structurally spliced by the longitudinal stringer members. The lightweight doubler is attached to, and covers, at least a portion of the outer surfaces of the frame and stringer members. The laminate sheet, comprising alternating layers of metal and composite, is disposed over and attached to the lightweight doubler so as to form an outer skin of the fuselage strengthened by the underlying doubler, the frame members and the stringer members.
The stringer wall structure of each of the longitudinal stringers may include a flange defining the outer longitudinal surface. Also, the frame wall structure of each of the frame members may include a plurality of flanges, each of the flanges defining a respective one of the outer circumferential surfaces. Each of the flanges of the wall structure overlaps a portion of the flange of each of the respective pair of longitudinal stringer members. Preferably, the overlapping flange portions, the lightweight doubler and the laminate sheet are attached together using a fastener. More preferably, the fastener is a knife-edge, countersunk fastener, such as a rivet.
Optionally, the laminate sheet may be bonded to the lightweight doubler using an adhesive layer, such as a corrosion inhibiting adhesive layer. Preferably, the surfaces of the doubler and the laminate skin are anodized before application of the adhesive layer.
Preferably, the metal layers of the laminate skin are aluminum layers and the composite layers are a mixture of fiberglass and epoxy. In addition, the doubler is preferably constructed of a lightweight aluminum.
The present invention has several advantages. The relatively thick and hard aluminum doubler reduces the stresses around the fasteners in the skin assembly. Such a reduction in the fixation stresses allows the use of a wider range of fastener types, such as the knife-edged, countersunk rivets illustrated herein that have excellent durability. Further, the doubler is easily tailored to local loading conditions (unlike most laminate skins) and is an independent, fail-safe member working with the frame and/or stringer. The doubler also allows the laminate skin to have a constant gauge, or thickness, even in areas having cutouts for receiving windows or areas requiring the use of fasteners. A constant gauge skin is more cost-effective than a customized laminate skin requiring increased thickness in areas around fasteners or cutouts. The combined use of the bond layer and the fasteners results in an improvement in fuselage strength and reliability over the use of bonding alone to attach structural members directly to a laminate skin. In addition, the configuration of the stringer members and the frame members provides for continuous load paths along the length of the stringer members and the circumference of the frame members. The result is an overall increase in the strength of the fuselage without a significant increase in weight. Such an increase in the strength of the fuselage provides the option of using smaller stringer and frame members to reduce the weight of the fuselage.